Variable Density Heliostat Field Layout

ABSTRACT

A heliostat array having a variable field density for use in a concentrating solar power (CSP) plant. Heliostats are arranged into subgroups and configured to track the sun and reflect light to a receiver tower. Heliostats are deployed onto structures, wherein the structures can be arranged in rows separated by a service gap. The structures can comprise cross members that can be varied in size. By altering the size of cross members in a structure, heliostats in one row can be deployed farther apart or closer together than heliostats in a different row. Heliostat field density can vary with distance from the receiver tower, wherein heliostats close to the receiver are more tightly packed and heliostats further from the receiver are spaced farther apart. Heliostat subgroups can exhibit variable heliostat density using one or more of the following features: variable spacing of heliostats within the same row, variable spacing of heliostats mounted to the same structure, or by varying the width of the service gap between rows. The result is a field configuration that reduces the blocking and shading of heliostats by their neighbors.

CROSS-REFERENCES TO RELATED APPLICATIONS

The present application claims priority to and the benefit of U.S.Provisional Patent Application No. 62/027,735, filed on Jul. 22, 2015,the entire disclosure of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

This disclosure relates generally to a field layout of heliostats in aconcentrating solar power system. In particular, the invention relatesto an improved configuration of heliostat support structures, whereinthe heliostats are distributed in a varying density depending on theirproximity to a receiver.

In Concentrating Solar Power (CSP) plants, arrangements of heliostatsreflect sunlight toward a receiver mounted atop a tower containing aworking fluid. One type of receiver transfers incident radiant energy tothe working fluid to produce high-pressure, high-temperature steamthrough the means of a heat exchanger or a phase change of the workingfluid itself. The working fluid can be water, air, or a salt materialheated to a molten state. The output steam can facilitate a variety ofapplications, such as electrical power generation, enhanced oilrecovery, and desalination. Heliostats are generally mounted on theground in an area facing or surrounding the receiver tower. Eachheliostat has a reflector: a rigid reflective surface, such as a mirror,that tracks the sun through the actuation of a heliostat drive mechanismabout at least one axis. Sun-tracking involves orienting the reflectorthroughout the day so as to optimally redirect sunlight from the suntoward the receiver and maintain the desired temperature of the workingfluid.

Arrays of heliostats can be arranged into a plurality of subgroupscomprising a field. The subgroups are configured to provide a preferredorientation that facilitates efficient land usage, optimizes the amountof solar flux delivered to the receiver, minimizes the blocking of outerheliostats by inner heliostats, and balances total system costs. Oneapproach to constructing a heliostat field is to utilize a small amountof comparatively large heliostats (e.g., having an area between about 50m² and 150 m²). In such a power plant, having a fewer number ofheliostats can necessitate the manufacture of very precise, and thusvery expensive, components for the positioning of the reflectivesurfaces. Another approach, however, is to use a large amount ofcomparatively small heliostats (e.g., having an area between about 1 m²and 10 m⁷), such as with reflective surfaces that measure between about1 m and 3 m on each side. Such an approach can be more efficient atredirecting sunlight because there are more individually adjustablereflective surfaces. In addition, smaller heliostats can be cheaper toproduce and easier to assemble, decreasing installation time andoperations costs. The use of smaller heliostats does present its own setof challenges, however. In order to be cost effective, heliostats mayrequire framed bracing to stabilize them when exposed to wind. Tominimize the amount of bracing used, it is preferred that heliostats bedeployed close together in groups or rows having shared supportingmembers. This increased packing density can result in some heliostatsblocking and shading other reflectors depending on the sun's position ortheir distance from the receiver. For a large arrangement having evenlyspaced heliostat rows this issue is exacerbated for the rearmostreflectors that must lower their angle of attack to reflect light ontothe receiver. When a heliostat mirror is shaded or blocked, this lowersthe amount of solar flux that can be delivered to the receiver. Theplant must compensate for this reduction in incident energy by focusingmore heliostats onto the receiver tower. Reducing the shading onheliostats therefore minimizes the amount of reflector area that mustface the receiver in order to deliver the same amount of requisite flux.

One solution to this problem is to adequately space the heliostat rowsfrom each other and to widen the gap between adjacent heliostats in arow to prevent blocking and shading of proximate reflectors. However,because the angle of reflection of light from a reflector to thereceiver is lower the further the heliostat lies from the base of thetower, the optimal heliostat layout that minimizes blocking and shadingwhile conserving land use is not the same at the front and rear sectionsof a large array. One option is to vary the density of heliostats in thearray such that heliostats furthest from the receiver are spaced thefurthest apart. This would have the side-effects of increasing the cablelength required to supply power and communication distribution pathwaysto the rows as well as increase the size of brace members needed forstructural support. The height of the receiver tower could be increasedto present a more accessible target for rearmost heliostats, but thissubstantially increases receiver tower costs for very large heliostatfields. Therefore there exists a need to achieve variable heliostatfield density while limiting overall cable length, bracing structureusage, arid receiver tower height.

SUMMARY OF THE INVENTION

An improved heliostat field layout for a concentrating solar power fieldis described herein, wherein the heliostat field layout comprisessubgroups of heliostats having a variable field density. A heliostatsubgroup comprises a plurality of heliostats arranged in predeterminedconfiguration. In the preferred embodiment of the present invention theheliostats are deployed in rows, though they can also be deployed inother suitable arrangements, such as in a radial pattern. The density ofa heliostat field is considered to be the number of heliostats deployedwithin a defined land area. Field density can be altered by varying thedistance between adjacent heliostat rows or by varying the spacingbetween heliostats within the same row.

A heliostat comprises a drive assembly that can actuate about two axes(for example, azimuth and elevation) and a reflector mounted to thedrive. Drives are operated via control boards installed within the driveassembly. Commands issued to individual heliostats originate from acontrol system within a plant network and are delivered via acommunication distribution topology comprising inter-drive connectorcables. Each heliostat is installed onto the post of a structurefastened to the ground. Heliostat structures can comprise a single postor can comprise multiple posts for mounting multiple heliostats.Multiple structure posts can be linked via cross members and can bearranged such that the posts and cross members form a geometric shape.Cross members can be connected to each other along the span between twopost members at one or more points. Additional cross members can also beincluded to provide additional support for the structure.

In an example of a preferred embodiment of the present invention, theheliostat structure comprises three post members, wherein each postmember is connected to the other two posts via at least two crossmembers to form a triangle. A single triangle structure is called a“pod”. The triangular arrangement allows the structure to sit onirregular ground surfaces without a loss of contact. The trianglestructures are arranged in a hexagonally-packed layout wherein adjacenttriangles alternate their orientation such that two adjacent triangularstructures form two parallel rows comprising three heliostats each.Multiple adjacent triangle structures in a line thereby always establishtwo rows of heliostats, collectively called a “row-pair”. Adjacentheliostat row-pairs can be separated from each other in the heliostatarray by a gap. This gap can be sized so as to provide access formaintenance vehicles or service crews for repair and cleaning ofheliostats.

Adjacent heliostats in a row are connected to each other via inter-drivecables, wherein the inter-drive cables facilitate both communication andpower-delivery via constituent wiring. Each heliostat control board isintegrally connected to an inter-drive cable having a male cable end andan inter-drive cable having a female end. The male or female end of oneof the heliostat's inter-drive cables can be connected to the compatiblecable end of an adjacent heliostat to connect the drives in series.Cable trays can be situated at one or both ends of the row-pairs orbetween heliostats in a row-pair to rout power to heliostats fromcorresponding buses and connect heliostats to the plant network.Communication interface modules can be mounted to the cable trays. Thecommunication interface modules are capable of interfacing withinter-drive cables and can serve as intermediaries between the plantnetwork and the heliostats. The cable trays can comprise field cablescapable of interfacing with the communication interface modules toprovide power and communication pathways to and from the plant network.Inter-drive cables can connect heliostats in the row-pairs to thecommunication interface modules mounted on the cable trays.

Heliostats can have their inter-drive cables daisy chained together,such that the same transmission line supplies power and facilitates datathroughput to multiple units in a single subgroup. Heliostats inalternating and adjacent rows can also be connected to each other viainter-drive cables to create redundant power and communicationtransmission pathways. For example, communication interface modules canbe connected to the nearest heliostats in separate, adjacent row-pairswhile the heliostats at the ends of adjacent rows in the same row-paircan be connected to each other, thereby creating a power andcommunication transmission loop. In the event that a single component inthe loop malfunctions, power and communication to an entire subgroup ora substantial portion thereof can still be maintained.

As described previously, a heliostat array exhibiting variable fielddensity is advantageous for limiting receiver height and reducing theblocking of heliostats by their neighbors. Designing the heliostatstructures to accommodate variable heliostat density allows forcustomization of the array in response to various plant siterequirements, such as those impacted by the terrain or topography. Onemethod of varying the field density is to lengthen or shorten theservice gap between heliostat row-pairs. This increased distance canhelp minimize blocking and shading effects on adjacent heliostat rows.

An additional method of varying the heliostat field density is tolengthen or shorten the distance between heliostats in the same row byaltering the dimensions of the structure cross members. Heliostatstructures in the same or different subgroups may have the same ordifferent cross member sizes, with the size of the cross memberscorresponding to the distance of the heliostat row-pair from thereceiver tower. The cross members can have a fixed length or cancomprise retractable or elongating members that are adjustable duringfield deployment. If the triangle heliostat structures are widened in afirst direction to increase the distance between heliostats in a row,this can reduce the width of the triangle structures in a seconddirection. Thus even if the number of heliostats in a row decreases dueto reduced field density, the drives in the row can still be linked withthe same total length of inter-drive cable, saving on material costs andeasing field deployment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A, 1B and 1C are a detailed view of a heliostat drive installedin a triangle heliostat structure.

FIG. 2 is an overhead view of a heliostat array exhibiting a fixedheliostat density. The array comprises a subgroup of triangle heliostatsupport structures arranged in row-pairs, wherein each row-paircomprises two rows of heliostats linked via inter-drive cables.

FIG. 3 is an overhead view of a heliostat array exhibiting variableheliostat density. The array comprises a subgroup of triangle heliostatsupport structures arranged in row-pairs, wherein each row comprises tworows of heliostats linked via inter-drive cables.

FIG. 4 Is a zoomed-out view of a full heliostat field in a CSP plant.The heliostat field is comprised of multiple heliostat subgroups, eachsubgroup comprising a plurality of heliostat rows. The heliostatsubgroups are arranged in a hexagonal pattern surrounding a centralreceiver tower. Some of the heliostat subgroups may exhibit a fixedheliostat density while others exhibit a variable density, wherein theheliostat density of a subgroup is a function of its location andorientation with respect to the receiver.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In FIGS. 1A, 1B, and 1C the triangle heliostat structure [101] iscomposed of three structure posts [110] and six cross members [111]arranged in three pairs. The cross members [111] are attached to eachstructure post [110] with a fastener. Cross members can be composed of avariety of materials, including wood, metal, or plastic. If multiplecross members [111] are used to connect two structure posts [110], thepair of cross members can be fastened together at one or more locationsalong their length. A heliostat drive [130] is installed in eachstructure post [110]. The drive comprises a capsule [150] that houseselectronics and facilitates the egress of inter-drive cable connectors[170]. The drive is mounted to the structure by physically inserting thedrive post [160] into the structure post [110]. One or more stakes [135]are placed on each post [110] to provide anchoring of the heliostatstructure [101] to the ground. The structures can also be fastened tothe ground by a number of means including, but not limited to, adhesivesor weights.

Inter-drive cables connect heliostats together by interfacing with therespective inter-drive cable connectors that are attached to the drivecontrol boards. The cables thereby facilitate the distribution of powerto the heliostat drives and create data communication pathways formonitoring, feedback, and control. Cables between heliostats in the samepod can be attached to the triangle structures with the use of one ormore fastening devices such as twist ties, clamps, clips, wire,adhesive, or other suitable methods. The fastening devices serve tominimize the movement of the cables due to wind and to provide strainrelief. Inter-drive cables are routed between heliostats in adjacentpods or in adjacent row-pairs by hanging the cables between structuralmembers or by positioning them on a support member. Examples of supportstructures can include a wire, a rigid member, a flexible member, aslot, or an enclosed tube. A support may be used to keep the cableraised off the ground to provide additional strain relief. Cables can berouted along the outside of the surface of the heliostat trianglestructures or routed within the internal surface of the post and crossmembers. Cables can be connected to each other in a variety ofconfigurations. For example, chained inter-drive cables can be run alonga single row, looped around the end of the row, and then extended toconnect to the nearest structure of an adjacent row. Alternatively,chained inter-drive cables can be run along the cross members of asingle triangle heliostat structure and then extended to connect to thenearest adjacent structure of the same or adjacent row.

In FIG. 2, a subgroup of heliostats [201] are arrayed in a plurality ofpods [202], wherein each pod comprises three heliostats [203] mounted toa triangle support structure [204]. A series of adjacent heliostat podsdefine row-pairs [205], wherein each row-pair comprises two parallelrows of heliostats [206]. Adjacent heliostats in a row are connected toeach other via inter-drive cables [207]. In the present figure thestructures have the shape of equilateral triangles, although they canalso have an isosceles shape. The space between the closest rows of twoneighboring row-pairs forms a service gap [208]. The service gap must bewide enough to accommodate the movement of workers and cleaning vehiclesfor the upkeep and maintenance of the heliostat field. In the presentfigure the service gap is of an equal distance between all row-pairs.

In FIG. 3, a subgroup of heliostats [301] are arrayed in a plurality ofpods [302], wherein each pod comprises three heliostats [303] mounted toa triangle support structure [304]. As before, a series of adjacentheliostat pods define row-pairs [305], wherein each row-pair comprisesto parallel rows of heliostats [306]. Adjacent heliostats in a pod areconnected to each other via inter-drive cables [307]. The nearestadjacent heliostats in neighboring pods within the same row-pair arealso connected to each other with inter-drive cables. The space betweenthe closest rows of two neighboring row-pairs forms a service gap [308].In the present figure the service gap between each row-pair has adifferent width, wherein the width of the service gap increases thefurther the row-pair is deployed from the receiver [not shown].

Additionally in FIG. 3, the length of the cross members of thestructures have been altered to widen the lateral distance betweenheliostats in the direction of the row. This has the effect of changingthe shape of the structures to isosceles triangles. In order to vary thefield density of the array to limit blocking and shading of neighboringheliostats the triangle configuration may be different between differentsubgroups or may be different within sections of the same subgroup. In apreferred embodiment, heliostats proximate the receiver tower can bedensely packed on equilateral triangle structures as in FIG. 2.Heliostats further from the receiver can be mounted onto isoscelestriangle structures as in FIG. 3. In the isosceles triangleconfiguration, the widest angle between two cross members can be between60 and 130 degrees.

In FIG. 4, an example of a heliostat array according to one embodimentof the present invention is depicted. The array [400] comprises aplurality of heliostat subgroups [411-417] surrounding a centralreceiver tower [418]. The array

is configured in a hexagon shape having a variable heliostat fielddensity. The plurality of heliostat subgroups comprises a fixed subgroup[411] closest to the receiver tower [418], wherein the first subgrouphas a hexagonal shape and comprises heliostats deployed in a constantfield density in the manner shown in FIG. 2. The plurality of heliostatsubgroups additionally comprises variable subgroups [412-417], whereinthe variable subgroups have a trapezoid shape and comprise heliostatsdeployed in a variable field density in the manner shown in FIG. 3.Within a subgroup, the heliostat rows and row-pairs are parallel to oneanother. Heliostat row-pairs in subgroups immediately opposite the fixedsubgroup [411] are also parallel, while row-pairs in adjacent subgroupsare not parallel to one another. Heliostats in the fixed subgroup [411]can be mounted on equilateral triangle structures having equal crossmember lengths. Heliostats in the variable subgroups [412-417] can bemounted on isosceles triangle structures, wherein the width of at leastone cross member is longer than the other two cross members. The servicegap between row-pairs in the variable subgroups increases the fartherthe row-pair is from the central receiver. Cable trays can be located atone or both ends of each row or row-pair to connect the closestinter-drive cable end of one or more subgroups to the plant network andpower delivery systems. Cable trays can also be located betweenheliostats in a row to further segment a group of heliostats intoadditional subgroups. The cable trays can comprise communicationinterface modules that facilitate power and communication distributionbetween the heliostats in the row-pairs and the plant network.Heliostats in each row pair are spaced apart from each other such thatany two row-pairs of the same length among one or more subgroupsutilizes the same length of inter-drive cabling to interconnect theirconstituent heliostats.

Alternative embodiments of a variable density heliostat field may haveconfigurations in other shapes, such as circles, polygons, heptagons, orother suitable n-gons. For example, heliostat fields can be deployed ina radial configuration in concentric circles around a central receiver,wherein the heliostat rows and row-pairs are no longer parallel to eachother. Heliostats can also be deployed to face one or more receivertowers from one or more directions. All, or some, fraction of theheliostat subgroups can exhibit variable heliostat density using one ormore of the following features described herein: variable spacing ofheliostats within the same row, variable spacing of heliostats withinthe same pod, or variable spacing of the service gap width betweenheliostat row-pairs.

In yet another alternative embodiment, heliostat rows and row-pairs maybe arranged lengthwise pointing directly away from a receiver tower. Thetriangular structures of heliostat pods in these rows may be equilateraland all heliostat pods may have the same dimensions. Variable density ofthe heliostat field may be achieved by varying the longitudinal spacingbetween adjacent pods within a heliostat row or row-pair, wherein thespace between adjacent pods is determined as a function of theirdistance from the receiver.

In all of the preceding embodiments, varied spacing between heliostatsmay ensure that adjacent heliostats do not block reflected solar fluxfrom, or shade incident solar flux onto, nearby adjacent reflectors inthe same or adjacent rows or row-pairs. As distance from a receivertower increases, the angle relative to the ground at which heliostatsmust be oriented to reflect sunlight towards the receiver mustnecessarily decrease, increasing the chances for blocking or shading. Aheliostat field having a variable density as described in any of theaforementioned preferred embodiments will improve flux delivery in aconcentrating solar power plant and obviate the need for a tallerreceiver or shorter, less densely-packed heliostats.

Various combinations and/or sub-combinations of the specific featuresand aspects of the above embodiments may be made and still fall withinthe scope of the invention. Accordingly, it should be understood thatvarious features and aspects of the disclosed embodiments may becombined with or substituted for one another in order to form varyingmodes of the disclosed invention. Further it is intended that the scopeof the present invention herein disclosed by way of examples should notbe limited by the particular disclosed embodiments described above.

1. A heliostat array comprising: a plurality of heliostats mounted ontostructures; a plurality of heliostat subgroups comprising at least oneheliostat structure, wherein the density of heliostats varies throughoutat least one of the subgroups; and a receiver tower facing the pluralityof subgroups from at least one side.
 2. The heliostat array of claim 1,wherein each heliostat comprises a two-axis drive assembly and areflector mounted to the drive assembly.
 3. The heliostat array of claim2, wherein the heliostats in the array are configured to track the sunand reflect light from the reflector onto the receiver tower.
 4. Theheliostat array of claim 2, wherein each heliostat structure comprisesat least one structure post having an interface for attaching to thedrive assembly of at least one heliostat, wherein the structure postsare fastened to the ground.
 5. The heliostat array of claim 4, whereinadjacent structure posts are interconnected by cross members.
 6. Theheliostat array of claim 1, wherein the heliostats in each subgroup arearranged in rows, and wherein rows are separated by a service gap. 7.The heliostat array of claim 7, wherein the service gap betweenheliostat rows in the same subgroup or in different subgroups variesdepending on the distance of the rows from the receiver tower.
 8. Theheliostat array of claim 7, wherein the distance between adjacentheliostats mounted to the same structure varies depending on thedistance of the structure from the receiver tower.
 9. The heliostatarray of claim 7, wherein the distance between adjacent heliostats inthe same row varies depending on the distance of the row from thereceiver tower.
 10. The heliostat array of claim 7, wherein adjacentheliostats in row or mounted to a structure are connected to each othervia inter-drive cables that distribute both power and datacommunications.